Despite the biological necessity of vitamin B12 (cobalamin), its importance in phytoplankton ecology has been ignored for nearly three decades. Here we report strong and selective responses of phytoplankton communities to varying low levels (5--87 pM) of dissolved B12 in several coastal embayments. The ecological importance of this vitamin is inferred from observed declines in dissolved B12 levels as field populations of large (>5 ¿m) phytoplankton increased. In contrast, biomass of small (<5 ¿m) phytoplankton varied independently of B12 concentrations. These observations were corroborated by field-based nutrient amendment experiments, in which B12 additions stimulated growth of large phytoplankton taxa 6-fold over unamended controls. In contrast, small taxa (<5 ¿m) were largely unaffected. This study provides the first evidence of vitamin B12's influence on phytoplankton field population dynamics based on direct chemical measurements of cobalamin, and implicates B12 as an important organic regulator of photoautotrophic fertility in marine systems.